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Thought-Provoking, Conventional Wisdom-Questioning On-Line Essays

alignment

No; don’t laugh. I don’t mean I figured out how to tie my shoe or hit a forehand. I mean I think I discovered something that no one has though of before. At least, I’ve read a lot on this subject and I’ve never seen this mentioned by anyone else.

As you know from a previous post, I make telescopes. Because of the sensitivity of the particular lens I used for this one, I made both the objective mounting cell and the diagonal mirror collimatable. This allows me to adjust all the parts so that their optical axes are aligned precisely and I can see the sharpest possible images.

Most telescopes allow such adjustments, and there are lots and lots of instructions out there that tell how to collimate a telescope. If you’re so inclined, you can purchase gadgets ranging from a simple cylinder with a peephole and a crosshair to a complicated laser projector to help you collimate more easily and precisely. But how do you know when to collimate?

Poor collimation causes imperfect images in the eyepiece, and it can keep you from seeing objects that are as small and faint as you should. But there are plenty of reasons that your views may appear less-than-sharp, and most have nothing to do with optical alignment. Most often, turbulence at some layer of the atmosphere bends the light from the object and distorts the view. This is known as bad seeing. But maybe you have eyestrain; maybe you had beer with dinner and your optic muscles are too relaxed to focus well; maybe your telescope and eyepiece haven’t cooled down yet. Without being sure, should you pull out your collimation tools and take time tinkering that could be spent actually looking at the universe, even though it’s not tack-sharp?

The other night, I (to my knowledge) invented what I’m calling the Curmudgeon Test. When I pointed the red-dot rifle sight I use as a finder, I noticed that the star I sought was off to the side of the view in the main scope. That can happen. Who can always point within a fraction of a degree to a star? This particular star was a close double that I have, in the past, been able to see as two distinct stars. But this time, try as I might, I could not see them. I put it down to atmospheric turbulence and moved on. Saturn was up next, and once again the image was offset in the main scope after I lined up my finder carefully. I then had a hard time seeing details on the planet that I usually can. Well, it’s nota good night for high-power, finely-detailed observations, I thought, and sought out a galaxy instead. Bad seeing matters less on diffuse objects. I pointed the telescope to a nearby star to begin the step-by-step hop to the galaxy, and, wouldn’t you know it, the star was off-center in the eyepiece. But all of a sudden, the scales fell from my eyes as I realized that each object had been displaced by the same amount in the same direction!

This was no random error; the finder and the telescope were not pointing at the same spot in the sky. It could have been that the finder was mis-aligned, but I quickly ruled that out because it fastens securely to the telescope and is placed such that it never gets knocked around once set. This left collimation as the likely culprit. Even a small bump when putting away a telescope or shifting lawn equipment next to it in the shed can move a lens or mirror system enough that its axes no longer align. And when that lens or mirror tilts, it moves the image it produces in a corresponding direction. I checked it and, sure enough, the lens was canted slightly in the exact direction each star was off-center. With a couple of turns of a screw, I was back in business. To my relief, I noted that all of the details I had been missing were suddenly visible and sharp. The rest of the night was a true pleasure.

So, here is the test: If your telescope images are consistently displaced from those in a previously-aligned finder, check the collimation. Otherwise, don’t sweat it and look at something else, or try another night.

But at least you’ll know.

P. S. Only align your finder with a telescope in perfect collimation. Otherwise the world will little note nor long remember what I’ve said here today.